The Ginzei HaMelech is puzzled that Haman is able to restrain himself, a truly difficult task for even the most mature, righteous individuals, whom Haman is not. He answers that Mordechai’s status as an adviser to the king allowed Haman to restrain himself. His fear was self-motivated because killing another adviser of the king would worry the other advisers that they made a wrong decree (Taanis 29a), and would consider advising the king to rescind a recent decree, possibly the one ordering the mass-murder of the Jews.

As Rashi said above, Haman’s motivation was fear of retribution without the king’s explicit permission.

The reason for this is, ironically, his own participation in creating a law that women have to listen to their husbands (Esther 1:22). As the Malbim wrote there, the necessity to sign such a truism into law put into question any other otherwise immutable ideas. Even a minister being disrespected did not act without the king’s command.

According to Rebbetzin Heller, the reason Haman restrains himself is that a good retaliation requires careful planning.

The Baal HaTurim (on Bireishis 43:31) points out that the word vayisapek (“and he restrained himself”) is used only twice in TaNaCh. The first is where Yosef restrains himself from revealing himself to his brothers. Just as in the Yosef story, the predicament was precarious but ended positively, so too in this story, self-restraint leads to national joy for the Jews. The Ramban’s definition (on Vayikra 19:2) of becoming holy is to restrain oneself and not to over-indulge.